A fuel cell is a power generating system having low emission, high energy efficiency, and a low burden on environment. Therefore, in recent years, it is in the limelight again in a growing requirement for environmental protection. The fuel cells are power generating systems which are promising as a relatively small distributed type power generating facilities compared with the conventional large scale power generating facilities and as a power generating equipment for mobile units such as automobiles and marine vessel. Further, the fuel cell receives attention as power sources for small size mobile equipment or for portable equipment, and expected as alternatives a secondary battery such as a nickel-hydrogen battery, lithium-ion battery, and the like, or a battery charger, or to be mounted on a portable equipment such as a cellular phone and personal computer by using in combination with a secondary battery (hybrid)
In a polymer electrolyte fuel cell, in addition to the conventional polymer electrolyte fuel cell (hereinafter, may be referred to as PEFC) in which hydrogen gas is used as a fuel, a direct fuel cell directly supplying a fuel such as methanol also receives attention. Comparing to the conventional PEFC, the direct fuel cell has an advantage that an output is lower than that of conventional fuel cell, but a fuel is liquid and it doe not have a reformer and therefore energy density becomes high, and a running time of portable equipment per one charge becomes long.
In the polymer electrolyte fuel cell generally has a constitution in which electrodes of an anode and a cathode where a reaction supporting power generation occurs, and a polymer electrolyte membrane to be a proton conductor between the anode and the cathode constitutes a membrane electrode assembly (MEA) and a cell formed by sandwiching this MEA between the separator is an unit. Here, the electrode is composed of an electrode base material promoting the diffusion of a gas and collecting (supplying) electric power (also referred to as a gas diffusing electrode or a collector), and a catalyst layer which becomes a field of an electrochemical reaction. For example, in the anode electrode of PEFC, a fuel such as hydrogen reacts in the catalyst layer of the anode electrode and produces a proton and an electron, and the electron is transferred to the electrode base material and the proton is transferred to the polymer electrolyte membrane. Accordingly, the anode electrode is requires to have a good gas diffusing property, a good electron conductivity, and a good proton conductivity. On the other hand, in the cathode electrode, oxidizing gas such as oxygen and air reacts with the proton transferred from the polymer electrolyte membrane and the electron transferred from the electrode base material in the catalyst layer of the cathode electrode and produces water. Accordingly, in the cathode electrode, it is necessary to drain produced water efficiently in addition a good gas diffusing property, a good electron conductivity, and a good proton conductivity.
Further, among PEFC, in the direct fuel cell in which methanol is used as a fuel, different performance from that of the conventional PEFC in which hydrogen gas is used as a fuel is required. That is, in the direct fuel cell, in the anode electrode, a fuel such as an aqueous solution of methanol reacts in the catalyst layer of the anode electrode and produces a proton, an electron and carbon dioxide, and the electron is transferred to the electrode base material, the proton is transferred to the polymer electrolyte membrane, and the carbon dioxide passes through the electrode base material and released out of the system. Therefore, in addition to characteristic requirement of the anode electrode of the conventional PEFC, the permeation of fuel such as an aqueous solution of methanol and a discharging property of carbon dioxide are required. Furthermore, in the cathode electrode of the direct fuel cell, in addition to the same reaction as that in the conventional PEFC, a fuel such as methanol permeated through the electrolyte membrane reacts with an oxidizing gas such as oxygen or air in the catalyst layer of the cathode electrode produces carbon dioxide and water. Accordingly, since produced water increases, it is necessary to drain produced water further efficiently comparing to the conventional PEFC.
Hitherto, perfluoro base proton conducting polymer membranes typified by Nafion (registered trademark, produced by Du Pont Kabushiki Kaisha) have been used as a polymer electrolyte membrane. However, these perfluoro base proton conducting polymer membrane had a problem that in a direct fuel cell, permeation of the fuel such as methanol is large, an output of a battery and energy efficiency are not sufficient. Further, the perfluoro base proton conducting polymer is very expensive since it uses fluorine.
None-perfluoro base proton conducting polymer membrane which is different from the conventional perfluoro base proton conducting polymer membrane, for example, various polymer electrolyte membranes formed by introducing an anionic group into a non-fluorine base aromatic polymer are proposed in U.S. unexamined patent publication 2002/91225, U.S. Pat. No. 5,403,675, and Journal of Membrane Science, Vo. 197, 231-242 (2002). However, in these polymer electrolyte membranes, there was a disadvantage that if an amount of anionic ion to be introduced is increased in order to achieve, water tends to be taken in, and a fuel crossover of methanol or the like is large. As an improvement of this defect, a countermeasure that an amount of anionic ion to be introduced is decreased to reduce a fuel crossover is readily conceivable, but in this countermeasure, not only ion conductivity decreases, but also the polymer electrolyte membrane becomes hard when the polymer electrolyte membrane is used as a membrane electrode assembly, and therefore adhesion of the electrolyte membrane to the electrode becomes insufficient, and as a result of this the ion conductivity is reduced and performance of a fuel cell becomes insufficient. That is, when the electrolyte membrane has high heat resistance and high tensile elastic modulus, voids tends to be produced between the fine surface of a catalyst layer and the electrolyte membrane since the membrane is hard and is hardly softened even though it is possible to attain compatibility between a low fuel crossover and a high ion conductivity. Accumulation of bubbles of air or carbon dioxide in the voids also becomes a large resistance to ion conduction and performance of a fuel cell becomes insufficient.
As a countermeasure against these problems, for example, a method of interposing a substance having an ionic group between the electrolyte membrane and the electrode is proposed in Japanese Unexamined Patent Publication No. 59-209278, and Japanese Unexamined Patent Publication No. 4-132168.
In Japanese Unexamined Patent Publication No. 59-209278, a method in which polymer acid in paste form is applied onto the surface of a catalyst layer described in an example of the invention, and olefin electrolyte such as polystyrene sulfonic acid and polyethylene sulfonic acid is used as polymer acid. However, a specific method of forming paste or materials required for forming paste is not disclosed. And, durability of a material used is insufficient.
In Japanese Unexamined Patent Publication No. 4-132168, a method in which a perfluoro base proton conducting polymer is applied to an electrode and dried, and then the electrode and the membrane are unified by hi-temperature press, and monomer composition solution such as sodium polystyrene sulfonate and hexaethylene glycol dimethacrylate, a crosslinking agent, is applied to an electrode, and this electrode is joined with a electrolyte membrane, and joined one is heated and pressurized for 1 hour or more to combine the membrane and the electrode into one sandwiching a crosslinked polymer of the monomer is exemplified.
However, in these methods, since it takes much time to join the electrode and the membrane, or a temperature of about 150° C. is required, the monomer and the solution are unnecessarily permeated into the electrolyte membrane, and the has an adverse effect on the electrolyte membrane's effect of suppressing a fuel crossover and the ionic conductivity, and a fuel cell with high output density cannot be obtained. Further, materials described in these references is insufficient in that durability of adhesive layer between the electrode and the electrolyte membrane is insufficient a fuel cell with high output density cannot be obtained when the material is applied to a direct fuel cell using a fuel such as methanol.